    1. Golomb, S. W. et al, Digital Communications With Space Applications, Prentice Hall, Englewood Cliffs, N.J., 1964.    2. Titsworth, R. C., “Optimal Ranging Codes,” IEEE TRANS. SPACE ELEC. TELM., March 1963.    3. Dixon, R. C., Spread Spectrum Systems, John Wiley and Sons, Inc., New York, N.Y.    4. Harris, R. L. editor, Introduction to Spread Spectrum Techniques, Ministry of Defense, Christchurch, Hampshire, UK    5. Schilling, D. L., U.S. Pat. No. 5,260,967, Nov. 9, 1993    6. Magill, D. T., U.S. Pat. No. 6,049,576, Apr. 11, 2000.
Spread spectrum (SS) communications is presently being used for a number of commercial applications and is expected to proliferate as the demand for untethered communications increases.
The SS User Terminals (UT) must achieve time and frequency synchronization with the received signal in order to despread and demodulate the signal data. This generally involves a two-dimensional search in time and frequency. Typically in a direct sequence system, a “sliding correlator” dwells at a selected frequency until the time uncertainty has been fully searched and is then stepped to another frequency if the signal was not found.
Considerable work has been done to formulate “acquirable” codes with characteristics that minimize the time required for the search process. The JPL component codes (1,2) are a well known example.
Dixon (3) discusses using a special code sequence at the beginning of a transmission as a “synchronization preamble.” This is typically a short sequence that is repeated multiple times.
The Outbound (OB) link in a star configured network such as a cellular or satellite mobile telephone network is transmitted continuously and must be readily acquired by UT's at any time. In this case, it makes sense to introduce a synchronization sequence that is repeated periodically as part of a data framing structure.
The length of the sync sequence is generally determined by the amount of signal energy required for reliable signal detection and synchronization at a reasonable power level (the available signal energy is proportional to sync sequence length). However, if the sync sequence is constructed as a PN sequence, acquisition time generally increases with burst length. If matched filter detection is used, the filter bandwidth decreases with burst length, and more frequency bins must be searched.
One way to meet the sync energy requirement without using a code of excessive length is to form the required length sequence by repeating a short sequence several times. Davies (4) comments that once the short code is acquired, the time uncertainty is equal to the period of the code. The code itself may then be modulated with a pattern, each bit having the duration of the code, whose detection at the demodulator is the signal to make a transition to a different, longer, PN code. Thus, we have a synchronization sequence composed of a short, high rate code modulated by a low rate sequence. Schilling (5) suggests a sync code generator running at the data clock rate Mod(2) added to a sync chip code at the chipping rate. Magill (6) suggests a construction which employs Neuman-Hofman (N-H) codes for the high and low rate sequences to give desirable correlation properties.
A matched filter may be employed to detect the short high rate code. In this case, a correlation peak occurs every code period. Noncoherent post detection combining (PDC) can be used to enhance the output SNR over the length of the sync sequence. In this scheme, the desired correlation peak at the PDC output is only slightly larger than the adjacent peaks that are offset by a multiple of the code period as shown in FIG. 1. For example, if the sync code consists of 24 repetitions of a short code, the maximum correlation peak, which occurs when the sync burst is property aligned with the PDC time window, is less than 0.4 dB above the adjacent peaks. Thus, it is difficult to select the desired peak even at good signal to noise ratios. In this case, an ambiguity of an integer number of code periods exists. At this point, a coarse track operation could be implemented (on the detected correlation peak), to refine the frequency estimate. A coherent matched filter for the low rate code could then be enabled to resolve the code period ambiguity.
Note that the bandwidth of the coherent matched filtering required for ambiguity resolution in this scheme is very narrow (24 times narrower than the short code matched filter). This requires that a very accurate frequency estimate be made before the sync burst can be fully acquired, and multiple frequency bins may have to be examined.